A) Field of the Invention
The present invention relates to a process for the preparation of an indigo compound important as a dyestuff. More specifically, the present invention is concerned with a process for preparing an indigo compound by reacting its corresponding indole compound, which is unsubstituted at the 2- and 3-positions thereof, with an organic perhydroxide in the presence of a specific catalyst.
B) Description of the Related Art
According to the process adopted these days for the industrial preparation of indigo, a salt of N-phenylglycine is formed using aniline and chloroacetic acid or aniline, prussic acid and formaldehyde as raw materials. The salt is subjected to alkali fusion at an elevated temperature, whereby the salt is converted to an indoxyl compound. The indoxyl compound is then oxidized with air to indigo. This process, however, is accompanied by the problems that it includes such many and extremely complex reaction steps and, moreover, requires potassium hydroxide and sodium hydroxide in large amounts, resulting in the consumption of large energy upon their recovery and reuse and the need for special equipment therefor. There is hence an outstanding desire for the conversion to a simpler process.
Takaki, one of inventors in the present application, and his colleagues disclosed in Japanese Patent Application Laid-Open No. HEI 1-215859(A) a process for the preparation of an indigo compound, which features reaction of an indole compound which is unsubstituted at the 2- and 3-positions with an organic hydroperoxide. They also disclosed in Japanese Patent Application Laid-Open No. HEI 3-768(A) another process for the preparation of an indigo compound, which features reaction of the same indole compound with the same oxidizing agent in the presence of a metal compound catalyst which catalyzes the oxidation of the carbon atom at the 3-position of the indole compound, and/or in the presence of an additive. The yield of the indigo compound and the reaction velocity were, however, still not sufficient even with these processes.